Sybex CCNA Chapter 3: Subnetting, VLSM and Troubleshooting Instructor & Todd Lammle

Chapter 3 Objectives The CCNA Topics Covered in this chapter include: -Subnetting basics -How to create subnets -Subnet masks and CIDR Class C subnetting Class B subnetting VLSM Summarization Troubleshooting IP addressing

Subnetting Basics Benefits of subnetting include: –Reduced network traffic –Optimized network performance –Simplified management –Facilitated spanning of large geographical distances.

How To Create Subnets Take bits from the host portion of the IP address and reserve the to divine the subnet address.

Understanding the Powers of 2

Subnet Masks Used to define which part of the host address will be used as the subnet address. A 32-bit value that allows the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion.

Default Subnet Masks

Classless Inter-Domain Routing (CIDR) Used to allocate an amount of IP address space to a given entity (company, home, customer, etc). Example: /28 The slash notation (/) means how many bits are turned on (1s) and tells you what your subnet mask is.

CIDR Values

Subnetting Class C Addresses In a Class C address, only 8 bits are available for defining the hosts. Remember that subnet bits start at the left and go to the right, without skipping bits. This means that the only Class C subnet masks can be the following: Binary Decimal CIDR = 128 / = 192 / = 224 / = 240 / = 248 / = 252 /30

Class C 192 mask examples SubnetHostMeaning = 0The network (do this first) = 1The first valid host = 62The last valid host = 63The broadcast address (do this second) SubnetHostMeaning = 64The network = 65The first valid host = 126The last valid host = 127The broadcast address

Class C 192 mask examples Subnet HostMeaning = 128The subnet address = 129The first valid host = 190The last valid host = 191The broadcast address Subnet HostMeaning = 192The subnet address = 193The first valid host = 254The last valid host = 255The broadcast address

Subnetting Class C Addresses – Fast Method Answer Five Simple Questions:

How Many Subnets? 2 2 = number of subnets.  X is the number of masked bits, or the 1s.  For example, in , the number of ones gives us 2 2 subnets. In this example there are 4 subnets.

How Many Hosts Per Subnet? 2 y -2 = number of hosts per subnet. Y is the number of unmasked bits, or the 0s. For example, in , the number of zeros gives us hosts. In this example, there are 62 hosts per subnet.

What Are The Valid Subnets? 256-subnet mask = block size, or base number. For example = is the first subnet. The next subnet would be the base number plus itself or 64+64=128, (the second subnet).

What’s The Broadcast Address For Each Subnet? The broadcast address is all host bits turned on, which is the number immediately preceding the next subnet.

What Are The Valid Hosts? Valid hosts are the number between the subnets, omitting all 0s and all 1s.

Variable Length Subnet Masks (VLSM)

Subnets with no VLSM applied

Subnets with VLSM applied

VSLM, Example 1

VLSM, Example 2

Which IP address will be placed in each router’s FastEthernet 0/0 interface and serial 0/1 of RouterB?

Answer

Summary Example

Why can’t the Sales LAN get to Server A?

Why can’t hosts in the Sales LAN get to Server B?

What can the host address be?

Which addresses can you assign to the hosts?

Which IP addresses can you assign?

Summary Go through all written and review questions Review answers in class 35